The present invention relates generally to the tentering of material, and more particularly to the tentering of a material which alternately and selectively moves in opposite directions through a tentering field.
The tentering of materials relates to the securement of a material along its edges for drying and/or stretching such material. In many cases, tentering is used to stretch the width of material to a predetermined width, maintain the width of the material, allow the width of the material to shrink to a predetermined width, or to simply prevent side to side movement of the material. Tentering is often accomplished in conjunction with other finishing processes, such as heating; and thus the structure and arrangement of tentering devices must often accommodate the practice of such additional finishing processes. In some cases, materials can be over five-hundred (500) inches in width. Therefore, when such material is finished on a finishing machine including, for instance, ovens to heat the material and tenters to tenter the material, the tentering device must be arranged with respect to the lower and upper ovens so that the ovens can be positioned as close to the material on either side thereof.
The fabrics and materials which are tentered as part of an overall finishing process may include papermaker's clothing and textile materials such as woven and non-woven fabrics. Papermaker's clothing encompasses forming fabrics used by paper manufacturers to form the surface of paper products, wet felt-base fabrics, and dryer fabrics such as heavy woven fabrics. The present invention relates to the tentering of all such fabrics and materials, though it finds particular application with respect to the tentering of papermaker's clothing.
Conventional tentering devices include spaced tenter rails which define a tentering area therebetween. A tenter chain having tenter pins, grippers, clips or the like to secure an edge of material is carried by each tenter rail. The chains and pins, grippers, clips or the like are driven in one direction only by a single motor which translates the power necessary to drive the chains by mechanical means, such as gears, pulleys, etc. In some cases, the tenter rail includes at one end a guide section which is coextensive with the remaining section of the respective tenter rail, but is pivotally attached thereto. Each guide section typically carries a portion of the respective chain and pins, grippers, clips or the like in order to guide material into the tentering device so that the material can be moved through the tentering area toward the opposite end of the tentering rail. The material to be tentered is moved into the guide sections by material feeding and take-up rolls arranged at either end of the tentering device. In the case an endless web of material, the material is loaded on and between a pair of spaced rolls (often referred to as the head roll and tail roll) to form a loop which is moved by rotation of the rolls so that either the top flight or bottom pass of the material is moved through the tentering area.
Such conventional tentering devices are shown in U.S. Pat. Nos. 1,732,089 to Honeyman, Jr.; 3,264,704 to Prottengeier; 3,430,310 to Richbourg; 3,932,919 to Hutzenlaub; and 4,639,984 to Langer. Further, in at least one case, individual motor drives are provided for each tenter rail of a tentering system for driving the chains in a single direction, as shown in U.S. Pat. No. 2,580,233 to Laurie which provides a synchronizing shaft to mechanically connect the motors for weft straightening. In U.S. Pat. No. 4,497,096 to Richter, both chains are mechanically driven in a single direction by a single motor at the exit end, though two motors are provided at the entrance end to remove slack in the tentering chains which occurs at the exit end, thereby avoiding the chattering of the tenter chains.
During most finishing operations, the material must be held between the spaced rolls under extremely high tension to yield a product of high quality. By reason of such high tension, the medial portions of the rolls deflect towards one another as the loop of material is moved in the machine direction at the speeds required for finishing. Thus, the circumference of the loop of material at the machine's center line, i.e., near or at the center of the fabric width, is less than the circumference at the edges of the fabric. This is particularly true where wide lengths of fabric are being run through the tentering device. The result is that upon each revolution of the loop of material, the cross machine direction (CMD), i.e., those yarns running in the cross machine direction, near the machine's center line will advance ahead of the CMD yarns near the edges of the material. Thus, the material is bowed in the machine direction of movement. This is referred to as a leading bow of the warp yarn.
One attempt to reduce or eliminate this leading bow has been to advance the speed of the tentering device with respect to the speed of the driving roll on which the material is held. However, doing so only affects on those portions of the material which are adjacent the edges. Thus, for wide materials, there is no compensation for the lead bow in the center of the fabric. The result can be a "gull" like appearance of the CMD yarns where the medial portion of the fabric has a leading bow flanked by lagging bows between the leading bow and the edges of the fabric.
It is therefore desirable to provide a tentering apparatus and method which can eliminate or reduce the leading bow of the warp yarn without introducing a lagging bow or other infirmities which affect the integrity of the finished material. The present invention accomplishes these objectives, as well as several other objectives which will become apparent from the following.
Another shortcoming in connection with the finishing of materials by tentering in conjunction with, for instance, heating, is the period of time it takes to cool the material. After heating a material, it is often desirable to cool the material as rapidly as possible to improve the quality and characteristics of the finished material. In conventional finishing machines, the tentering device and ovens for heating are arranged closer to the head roll than to the tail roll, the tail roll being moveable depending upon the length of the material. Thus, there may be quite a distance between the ovens and the tail roll such that material moving from the head roll to the tail roll will cool only by exposure to the ambient temperature. Such cooling will occur at a relatively slow rate. The only options for more rapidly cooling material moving from the head roll to the tail roll using a conventional finishing machine would be to use external cooling apparatus. Of course, this can be expensive and difficult given the large size of the material and the structure and characteristics of finishing machines. The present invention addresses this shortcoming as well.
In addition, with respect to tentering devices arranged below the bottom pass of a continuous length of material looped on the head roll and tail roll, there is little clearance between the tenter pins, grippers or clips and the bottom pass of material. Therefore, care must be taken in loading and removing the continuous length of material on or from the rolls. Because of the large size of finishing machines and the structure of the mechanical drives used to drive the tenter chains, it is difficult to move the head roll and tail roll system and/or the tentering system with respect to one another. Heretofore, lower ovens arranged below the bottom pass of material have been elevated into position once the material is loaded, and lowered after completing the heating process. This allows the ambient air to cool all portions of the material, even the portions in the area of the oven. As stated above, such elevation of conventional tentering devices is not possible or at least difficult because of the large and intricate mechanical drives which translate the power needed to drive the tenter chains. The present invention also addresses this problem.
Further, some tentering systems provide one moveable tenter rail so that the width between the tenter rails can be adjusted for materials of different widths and, equally important, for adjusting the cross machine tension on material being moved through the tentering area. To adjust the width or tension, however, the entire moveable tenter rail must be moved such that the width at the first end is always equal to the width at the second end. While this may be acceptable for adjusting the system for materials of different widths, it may not be acceptable for adjusting the tension while material is moving through the tentering area. In the latter case, the tension on the material at the entrance of the tenter rails due to the sagging of the material, etc., may be less than the tension on the material at the exit. In any event, it may be desirable for other reasons to provide different tensions at different points on material as it is moved through the tentering area. The present invention also contemplates the introduction of different tensions on material moving through a tentering area, at least for a period of time.
Lastly, when the power to the tenter drive motors of conventional tentering devices fails, or the motor itself malfunctions, the tentering chains carrying the pins, grippers, clips or the like will stop moving because of the mechanical connections between the chains and/or motor which drive the chains. However, since the head roll is driven by a separate drive motor, the material may continue to rotate, whereby the material may be damaged along its ends where it is secured by the pins, grippers, clips or the like. The present invention also provides a safety feature so that the product being tentered is not destroyed should the motor malfunction.